mad是指发疯，mad money是年轻女子跟男朋友一起出去玩的时候自己身上藏的一点钱。万一她跟男朋友生气了，或吵架了，那她至少自己有钱叫出租车回家去。真实含义是私房钱；应急钱

【例句】
That 's such a beautiful dress. I'm tempted to use some of my mad money and buy it .
那件衣服太漂亮了，我真想用一些私房钱买下它。

But we were lucky - my wife remembered she had her mad money, so we were able to buy gas to get back home.
但是，我们很幸运，因为我太太记得她有一些准备急用的现金，所以我们能买了汽油，开车回家。